1. Field of the Invention
The present invention relates to an apparatus and method for detecting humidity in a microwave oven, and particularly, for reducing or eliminates the affect of microwave noise on an absolute humidity sensor and obtaining a stable output from the sensor.
2. Description of the Prior Art
Generally, a conventional microwave oven contains a sensor for detecting the cooking condition of food to perform an automatic cooking function as one of the essential functions for the microwave oven. An absolute humidity sensor of a thermistor type is generally used as a cooking-condition detecting sensor since it can obtain an output linearly proportional to humidity generated from cooking food irrespective of any surrounding changes. But, microwaves generated by oscillating a magnetron which enter the absolute humidity sensor act as noise and cause errors in the output of the humidity sensor, thus lowering its reliability as a sensor.
In order to remove microwave noise introduced into the sensor in the above-mentioned manner, prior microwave ranges employ expensive parts for shielding noise such as heat-resisting shield wire, ferrite rubber, etc., and thus manufacturing cost is very high.
FIG. 1 is a diagram of a whole microwave oven to which a prior humidity detecting apparatus is applied. A turntable 4 on which a container 18 for food 3 is placed, is disposed at the lower side of a heating room 2 of the microwave oven 1. A driving motor 5 for the turntable is installed below the turntable 4. The heating room 2 has an air suction opening 6 and a microwave guide pipe 7 on one side wall and has an exhaust opening 8 for air and vapor. In the vicinity of the air suction opening 6, there is magnetron 9, a fan 10 and a fan motor 11 for cooling the magnetron and blowing air into the heating room 2, and a driving section 13 for driving the turntable driving motor 5, magnetron 9, fan motor 11 and a magnetron oscillating high voltage transformer 12. Also, in the vicinity of exhaust opening 8, there is provided a humidity sensor 14, and a sensed signal processing section 15 for converting a variation of resistance values of humidity sensor 14 into a variation of voltage values and providing an output as information of cooking progress.
A controller 17 controls the heating of food 3 through driving section 13 according to the cooking progress information from the sensed signal processing section 15 and a key signal from a key matrix 16. A lamp 19 turns on during the cooking operation and a user can see food 3 in the heating room 2.
The operation of such a microwave oven is explained hereinafter.
In FIG. 1, when a user operates the key matrix 16 and selects an automatic cooking function, the controller 17 senses this selection and drives the turntable driving motor 5, the magnetron 9 and the fan motor 11 through the driving section 13, thus the food 3 in the heating room 2 starts to be heated. At the same time, outside air introduced via an inlet port 21 by means of a fan cools the magnetron 9, and a portion of the air is then directly exhausted to atmosphere via an outlet port 20 and the rest supplied into the heating room 2 via the air suction opening 6. The air in the heating room 2 is exhausted to atmosphere via the exhaust opening 8 together with the vapor generated from the heated food 3. The vapor exhausted in this way passes by the humidity sensor 14 and changes the resistance value of the humidity sensor 14, such change being converted into the voltage variation by the sensed signal processing section 15 and supplied to the controller 17 as a humidity-sensed signal. The controller 17 analyzes the inputted humidity-sensed signal and judges the present cooking situation, performing automatic cooking by controlling the output of the magnetron 9 according to the present cooking situation.
FIG. 2 shows a prior humidity sensor 14 which detects humidity, and a sensed signal processing section 15 for processing the sensed signal. The humidity sensor 14 comprises an open-type thermistor TH1 and a closed-type thermistor TH2 which are disposed in the vicinity of the exhaust opening in the heating room. The two thermistors TH1 and TH2 form a bridge circuit together with two resistors R5 and R6 in the sensed signal processing section 15. The sensed signal processing section 15 also includes an operational amplifier OP1 resistors R1 to R4 and a diode D1 for comparing and amplifying the voltage variation outputted from the bridge circuit.
When the vapor is exhausted via the exhaust opening 8 in FIG. 1, the vapor is contacted with the open-type thermistor TH1, thus varying the inner resistance value of the open-type thermistor TH1. The resistance value variation varies the voltage balance of the bridge circuit TH1, TH2, R5 and R6, and the varied voltage value is supplied to the controller 17 as a humidity-sensed signal V out after being compared and amplified by the operational amplifier OP1. In this case, the closed-type thermistor TH2 is used for compensation of sensed temperature.
However, after oscillating the magnetron 9, a microwave noise according to the influence of the microwave leaking out exhaust opening 8, is introduced into the humidity sensor 14, lowering accuracy and reliability in the humidity sensing operation. Thus, there is a problem causing faulty functioning in the automatic cooking performance.
FIGS. 3A and 3B illustrate a prior humidity sensor having a shield structure for blocking microwave noise. As shown in FIGS. 3A and 3B, the sensor comprises an open case 23 which encloses the open-type thermistor TH1 and has a hole 22 for introducing the vapor therethrough, a closed case 24 which encloses the closed-type thermistor TH2, conducting wires 26 connected to leads 25 to which each thermistor TH1 and TH2 are attached, and ferrite rubber 28 and shield wires 27 which in turn cover the conducting wires 26.
But, the rubber 28 and shield wires 27 are very expensive, having a problem of raising the manufacturing cost. Also, with this shield structure it is still difficult to remove the influence of the microwave noise completely.